Windmills may be constructed using either a horizontal axis, as in most modem wind turbines, or using a vertical axis, as in the earliest windmills. The first vertical axis windmills are usually attributed the to Persians or to the Chinese during the period 500 to 900 C.E. These windmills were used to grind grain or to pump water. Today however, horizontal axis wind turbines are the dominant variety and are used to generate electricity. They are being installed in large numbers, particularly in Europe and are able to produce megawatt quantities of electricity. These large wind turbines are expensive, costing approximately $1 million dollars each in 2003 and, for this reason, are usually owned by utilities. The diameters of the newest machines are 328 feet, with towers of 200 feet or more. These machines are clearly not appropriate for a small farmer wishing to extract energy from the wind to produce electricity for domestic use, for net metering, or for sale to electric grids.
Early in their careers sailors learn to beware of an accidental gybe when sailing, in particular when “running” with the wind. The danger of an accidental gybe is given by the following quote from the classic book of sailing “Chapman's book on Piloting: Seamanship and Small Boat Handling, 60th Edition” by E. S. Malony: “Whenever jibing (gybing) or close to a jibe, watch for an accidental jibe—one for which the crew is not prepared. With inattentive steering the wind may catch the back side of the sail and throw the boom violently across the boat to the other side, risking serious damage to the rigging and to the heads of crewmembers.”
A vertical axis wind turbine would normally produce an uncontrolled gybe when the wind catches the back side of the sail. The energy of the wind in producing this uncontrolled gybe is normally lost and not used to add force to the rotation.